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Reaching clearance limit
For EASA, if possible.
If I'm IFR flight flying STAR and I receive shortcut to a point which is behind IAF but I'm not cleared for eg. ILS RWY 12 approach, what I'm supposed to do?
I would say that the point which I was cleared to is my clearance limit. I would say that I can't continue that approach because I wasn't cleared to. I guess this shouldn't happen but if it happens what is correct action for a pilot according to a law?
I think that for FAA land it is simply start to hold with standard holding pattern but I'm not sure if this is the case for EASA.

Are you assuming lost communications?

I think he means not a coms failure but where the controller is busy or preoccupied and you reach a clearance limit with no further instruction so whaddaya do on the spot.  A largely theoretical case but I suppose there are situations where it can happen.

@JohnK, I agree, but I don't like to presume.  The language used is also important pzag, if your clearance limit is the airfield and during the STAR you are told "proceed direct ABCD" your clearance limit hasn't changed.  If there were another point on the STAR after that I would continue to the last point on the STAR you were cleared for, (and query the controller) before I would hold.

@JohnK I agree with "largely theoretical" in that it's extremely rare to actually happen, but starting to get uncomfortably close to a clearance limit & hoping ATC's next transmission is for you, not as much. I think ATC is very interested in averting this sort of situation, but it's worth thinking through what's the best course if it does happen, IMHO.

No, I didn't assume lost communications. I know it is theoretical. This came up during ATC training. I work on a simulator for them as a pilot (I fly all the aircraft they learn to control) and they did exactly that and none really knew, not even instructors could agree, what to do next. The ATC in training sent me directly to point behind IAF but without any further clearance.

Hello pzag, welcome to aviation.stackexchange, nice question! Just to clarify: you were cleared for the STAR, and while on the STAR you were given a direct-to instruction to some waypoint between the IAF and the FAF, but not you were not given an approach clearance?

Do you remember how the direct-to was formulated?

It was exercise for radar/approach, so the ATC got aircraft on STAR. Then it was simply "proceed direct Y (which was the point behind IAF), descend to 5000 ft." So the aircraft ended up on approach but it wasn't cleared for the approach. Btw. yes, exactly as you said that.

First, I'd check the printed STAR: if it gives an instruction like "expect the ILS runway XX" then I'd go with that.
If you're the only airplane inbound to your destination, then holding might work, but in a busy terminal area where you are part of a long train of aircraft inbound on that STAR heading to an obvious runway, holding there outside of (but near) the IAF would create a dangerous situation for you & the next few aircraft in line, with the possibility of no easy place for the controller to vector you to.
I'd continue on final, squawk IDENT ("hey, look at me!" to get the controller's attention), and stay basically in line. If the controller doesn't want you flying the approach (say, he just got word of an unexpected runway closure) it's still easier for him to keep you at (or climb you back up to) altitude on that same groundtrack than to sort out you turning back toward inbound traffic or traffic on parallel finals.
Chances are good that he meant to clear you for the approach, or even thought he had cleared you for it, and once he can, that's what he'll do.
While I agree that "can't get a word in" isn't quite the same as Lost Comm, I'd defend Captain's Authority & doing the safest thing by continuing in, rather than holding, in the situation of a long string of arrivals.
That said, if you're inbound to a small airport with nobody close behind you, and you starting down final could create a conflict with some of the traffic that's making the frequency so busy, then that's an entirely different scenario, and holding in your protected airspace could well be safer than starting an approach into airspace that ATC needs to sanitize of other traffic.
I think that latter scenario is more what the "hold at your clearance limit" rule is based on.  In my flying (FAA-land, not EASA), I'm generally part of the long stream of traffic all inbound to the major airport, and we're typically given "expect the ILS 25L" by one or more arrival controllers while still on the STAR.

A “shortcut” does not change your clearance limit; it simply removes the portion of the route prior to the shortcut point.  Therefore, upon reaching that point, you would continue to the clearance limit.
For instance, if you were originally “cleared to X via direct A direct B direct C direct”, and ATC later says “cleared direct B”, your clearance limit remains X, but with the shortened route of “direct B direct C direct”.
Notice the distinction between “cleared to … via”, which sets/amends both the clearance limit and the route, and “cleared”, which amends only the route.  A couple tiny words make a huge difference in meaning.
However, since you were not cleared for an approach, you cannot descend below the last assigned altitude.  This leaves a question of what to do when you reach your clearance limit, but the pilot should have clarified with ATC long before that happens or, if unable, begin lost comms procedures.  And that will definitely get ATC’s attention!

The question wasn't about a "direct B", but about "direct Y", where Y is behind X. What should you do now? Turn around and fly back to X? Or hold at Y?

@Bianfable If Y isn’t already on your current route, I wouldn’t call that a shortcut.  Nothing in the Q leads me to think ATC intended to change the clearance limit.  Note they said “behind the IAF”, so it’s a fairly safe assumption that the clearance limit was the airport.

In FAA land, if it's not lost comm, fly your cleared route at your last assigned altitude to the clearance limit and then enter holding.


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Why does stall speed decrease when flaps are deployed?
I've been told that deploying the flaps on approach enables the following:

Potentially steeper approach
Slows down the plane
Lower stall speed

It's the last one that confuses me.

Given the equation for lift:
$ L = \frac{1}{2}C_l  ρ V^2 S$
with:

$L$ - lift
$C_l$- lift coefficient
$\rho$ - density of the air
$V$ - velocity
$S$ - surface area of the wing

When we deploy the flaps, we automatically change the camber of the wing which does the following:

Increases the surface area of the wing
Increases our current angle of attack

As a result of the change in surface area of the wing, we can decrease our V in the equation and generate the same amount of lift that was needed before flaps. Does that sound correct?
For most practical purposes, your average GA pilot doesn't deal with the AOA because it can't be readily calculated so the POH typically lists the stall speed Vs. However, as I understand it, stall speed is indirectly correlated with our plane's "critical angle of attack".
What I don't quite understand is, why exactly does our stall speed decrease when we deploy the flaps?
Is it because of what I outlined earlier, that we can produce an equivalent amount of lift at a lower speed because of the change in wing surface area?

If they didn't decrease stall speed, why bother having them? Points #1 and #2 can be achieved with flat-plate speed brakes.

I'm not sure I take your point? I'm not debating the fact that they decrease stall speed, I'm asking how it occurs.

Do you mind marking one of the provided answers? I am curious (and I think others as well) of what have helped you -if anything did- to understand.

@SteliosAdamantidis To be honest, that's one of the reasons I haven't marked an answer yet - they're both extremely helpful so I'm having trouble choosing between them. After rereading them again, I went ahead and marked an answer.

Your stall speed decreases because the $C_{L_{MAX}}$ of the wing increases with flaps deflected.
Deploying flaps increases wing camber and increases both $C_L$ and $C_D$ at the same AoA and airspeed. Depending on the type of flap, the critical AoA actually reduces: the airfoil with simple flaps deflected stalls at a lower AoA than without flaps.

Slats or leading edge flaps do not change the $C_L$ at the same AoA, but allow for a higher AoA and therefore higher $C_L$ before the airfoil stalls.

The flaps mostly increase the camber, which increases the Cl (albeit, with greater drag).
In quite a few cases (especially low-end GA aircraft) the flaps have little or no effect on the wing's surface area (the flaps are just a section of the trailing edge that swings downward on a hinge).

Even in those that do slide the flaps back on a track, the change in surface area is quite small compared to the change in camber (and with it, Cl).

To be explicit (and answer the question), the increase of $C_L$ provided by flaps extension allows to produce the same lift amount with a reduced speed for the same AOA, hence just before reaching the critical AOA, the aircraft can fly level at a lower speed.

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Can large airliners such as a 747 maneuver without electricity?
Small aircraft have the flight controls physically linked to control surfaces by some kind of cable and pulley system. For a large aircraft such as a 747, are the control systems mechanically aided and if so, can they be used without power?

Flight controls are pretty hard to break
For the 747 (at least through the -400) specifically, the controls are entirely hydromechanical, so yes, they will work without electrical power.  In general, all non-FBW airliners are this way.
FBW airliners vary, though.  All FBW Boeings and the first generation FBW Airbii (A320 family, A330/340) have hydromechanical backups to the FBW system.  The second generation Airbus FBW system, found in the A380 and A350, has abandoned hydromechanical backup controls in favor of independently powered electrical backup systems.
In addition, the chances of total electrical failure are extremely remote -- a failed engine will likely be windmilling (spinning from airflow through it) and thus turning its generator to provide some power, the APU has a generator on it in case all the engines seize up, and there is also a generator that is directly or indirectly powered by the Ram Air Turbine on airliners that use one (all non-reversion twins have one, and so do the A380 and the 747-8 -- earlier 747s don't, relying on windmilling engines for electrical and hydraulic power instead).  Finally, the aircraft has batteries that can provide a limited power reserve if all these other systems fail.  (FBW aircraft add to this with dedicated batteries for at least some parts of the FBW system.)
However, power loss is still a serious emergency
The main hazard posed by a severe electrical power failure, though, is trying to fly around in the clouds with one -- it can render you effectively "no-gyro", especially on older/smaller types that don't have a standby gyroscopic instrument with a dedicated power supply/internal batteries.  There have been cases where small airliners have gone down due to power failures -- while the flight controls were working fine, without power or a visible horizon, the pilots became spatially disoriented, and you can guess the rest.

I must ask: is Airbii supposed to be plural for "Airbus"?

@TomMcW -- indeed, it is :)

It depends on the airliner. In case of large airliners, the control surfaces are usually actuated by hydraulics, which in turn are controlled by (signals from) the Flight control Computer.
In case the electrical supply fails (extremely low likelihood actually), some aircraft have mechanical backup, while the others haven't got anything, apart from aircraft like 787, which have their controls actuated by electrical power and are dead once electrical supply fails.
However, the present trend is to move away from mechanical backup systems- so most modern aircraft are not controllable in case power supply fails. The evolution of Airbus backup controls illustrates this point:

• A320: full FBW controls, mechanical Back-up (Pitch Trim & Rudder)
• A340/A330: like A320, additional Yaw Damper to improve Dutch Roll
  damping even in Back-up mode (BYDU with hydraulic micro generator)
• A340-600: like A340 for pitch, Rudder becomes fully Electrical
  (BPS + BCM : Back-up Power Supply +  Control Module)
• A380: like A340-600 for Yaw control +  BPS + BCM also power

Electrical Pitch Back-Up (elevators) linked to side-stick
Electrical Roll Back-Up (ailerons) linked to side-stick
Pitch Trim (Wheel is replaced by Switches).

You can see that while older aircraft designs had mechanical backup systems, newer ones tend to have electrical ones. However, the probability of all the electrical supply failing at the same time is quite remote.

It would be worth adding that aircraft have a RAT to provide power in case other supplies fail

Specifically speaking, no they cannot. However, most planes carry a generator that is powered off the wind, and thus the planes forward momentum. This allows the plane to be flown without a functioning engine and power source.
This image shows the ram air turbine on F-105 Thunderchief fighter-bomber:

See also https://en.wikipedia.org/wiki/Ram_air_turbine

Did US Airways 1549 use the RAT?

@Firee: "When all of the
airplane’s electrical power is lost and the airplane has an airspeed greater than 100 knots (kts),
the RAT will automatically deploy and begin providing electrical power. The RAT was found in
the extended position when the airplane was recovered from the water.  ...  the pilots stated that they determined that electrical power was
established and, therefore, that the RAT did not need to be manually deployed. Further,
immediately after the loss of engine thrust, the captain started the APU. "

@Firee, yes, it did. Briefly. Then they started the APU, which provides more power, to restore complete functionality—in A320, the RAT does not power all three air data units due to which the aircraft is in alternate law when flying with RAT. Starting APU returned them to normal law, which turned out helpful during landing.

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Why do wings of passenger jet go dark during a flight?
Why do certain areas on passenger jet wings go dark during a flight? This effect is observed on high altitude where temperature is low. When airplane descends, wings return to their initial color.
Wing during a flight on high altitude.

The same wing before landing.

I've noticed this effect in several flights on different aircrafts. It is clear that these areas are covered with other coating, but what is the purpose of this?

Not sure but I think the original colour is as you see it at high altitude. When descending moisture condenses on the cold wing and changes the colour.

@DeltaLima I think you're right that it's about changes in the reflected light, the leading edge gets lighter too.

I see you have some dust in your camera's lens assembly.  You weren't using a Samsung S4 Zoom were you?

I was using Sony compact camera. So, not only Samsung cameras are apt to colect dust on its sensors. I've taken the camera to service shop a couple of times to remove dust, but it has restored quickly after that.

As DeltaLima said, the change in brightness is probably due to the difference in lighting. The first picture is above the clouds, so the air is clear and the light is primarily from the sun. The second picture is in the clouds, so the air is moist and the light is much more diffuse, coming from all directions. This diffuse lighting and the moisture on the wing surface help to reflect more light. You can see this in the already reflective leading edge as well. In the first picture, mostly the darker sky is reflected, but in the second picture, the brighter clouds are reflected. The reflected light will be more evident in the darker areas than lighter ones.
Regarding the materials, they represent different parts of the wing. The darker section represents the main structure of the wing, so the surface is made of metal for strength. The sections around it do not carry as much load so they are made of composites to save weight. The metal conducts heat better, so it will tend to condense more moisture than the composite section, as the aircraft is descending from cold, dry air into warmer, moist air.
The surfaces do seem to have different finishes. This is probably related to the different types of material there.
Those pictures illustrate your question very well. I don't specifically remember noticing this sort of thing myself, but I will see if I have any myself showing the same thing.
Also... anyone know what plane this is? The pointed ends of the shock bodies, the VG's, and the static wicks outboard of the aileron make me think a 737 classic with winglet retrofit.

Seems to be a Boeing 757 to me.

@fooot, thank you for the explanation. It seems very plausible, if skin on different parts of the wing is made of different materials. I'll try to make more pictures in a couple of weeks and post them if they bring new information to this question.

The aircraft is probably Boeing 737-500. http://imgur.com/WEuHEcF

Also, note that there's not only less moisture around to scatter the light - there's also less atmosphere! Compare with being on the ground (blue sky) with being in outer space (pitch-black sky, since there's no Rayleigh scattering). In fact, 80% of the earth's atmosphere mass is in the lowest layer (up to 12km). So by approximation, the sky above you while flying (which you can't see from the cramped windows) is about 20% as blue - i.e., rather dark, giving reflective surfaces a dark look.

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What runway materials reduce skidding in wet conditions?
What are the possible materials that can be used on a runway, so as to reduce the plane speed and certainly stop the plane while preventing skidding when the runway is wet?

Some related info here

This question has kept industry awake at night for ages now. A good start is here.
TL; DR: In addition to grooving, some runways (e.g. London City) apply a special surface material that is called a Porous Friction Course.

An alternative to grooving as a means of facilitating surface water dispersal is the laying of a Porous Friction Course (PFC) which allows water to pass vertically through the surface layer and then move horizontally clear of the runway beneath, but essentially parallel to the surface.

The trouble with all that is that if you test it, build it and then calculate performance taking credit for it, if it doesn’t work for real for whatever unknown unknown, you are likely to find out by having to remove an aircraft from the Thames river which is kind of frowned upon by most. You want to be a bit conservative with runway friction.

In Alaska where most GA pilots fly tail wheel bush planes, many airports have gravel runways because they work better in the snow and ice.  In fact many Alaskan pilots prefer landing on gravel runways even in good conditions because they don't like the fact that tires tend to skid when they first make contact with a hard surface runway.
That being said gravel runways don't tend to work as well for nose gear or heavy aircraft.  The bouncy nature of the gravel runway is bad for the nose gear and could result in a prop strike.  Also Alaskan planes will usually be equipped with larger wheel base tires that are designed for bush flying and work well for gravel runways.

Loose gravel can also be sucked up by the prop and thrown backwards, causing prop dings and dings on the tail. Large wheels on tail draggers keep the prop clear of gravel.  Tail still takes a beating.

I believe the most that is done is to have grooves on the runway surface to help with water runoff.
Airports also use vehicles with large brushes to help keep the runways clean.
Plane tires do not have sipes (the cuts that go across the tire tread) to disperse water out to the sides that car tires have to help with hydroplaning.

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Why are runway lights always on?
By driving often near my local airport, I deduced that runway lights are kept always on as soon as the day light or weather conditions require it.
I also suppose that it comes from safety reasons established in the past ad that's a worldwide standard.
what I don't understand is why, given nowaday's instrumentation both on ground and airborne, it's not possible to switch them off when no aircraft is ever going to even fly at cruise altitude around it. Here at LIBP (Labruzzo Airport, Italy) we hardly have more than 8 transits during winter so probably 80% of the energy spent by lights it's totally wasted.

In the US (don't know about other countries), runway lights are NOT always on.  Many smaller airports have pilot-controlled lighting.  Clicking your radio transmit button a specified number of times (usually 5?) in rapid succession turns the lights on, then a timer turns them off a few minutes later: https://en.wikipedia.org/wiki/Pilot-controlled_lighting

Some lights like a PAPI are always on even in the daytime.  Perhaps this is the light you are seeing?

The answer to the question lies in your statements

I deduced that runway lights are kept always on as soon as the day
  light or weather conditions require it

Typically when the visibility drops below a certain level airports will turn their lights on. You mention that the airport does not have a whole lot of arrivals but you would only be able to progressively turn them on and off if all the arrivals were planned. There are plenty of cases where they may not be, (here in the US GA traffic is one), emergencies, diversion, aircraft not operating on a flight plan etc. In that case its simple courtesy to leave them on.
In some cases the lights are actually part of the instrument system and the regulations may dictate that if the ILS is on so are the lights.
The lights may also not always be on, it may simply be that you happen to drive past them when they commonly are on. There are lots of variables here but different airports handle this differently. Here in the states smaller non-towered airports often have lights that are always off, a pilot can turn them on by keying the mic button on their radio 3 or 5 times. They are typically on a 10 or 15 minute timer. If you turn them on to far out and forget about it they may shut off on short final... Larger towered airports may keep some lights on and allow you to request others be turned on or off. This is only applicable if the tower is in operation. At night, if the tower closes they may simply leave the lights on as there is no way to remote key them.
At night, even in good weather (or especially in good weather) lights can help identify an airport from quite a distance. Since runways are lit in a predictable, standard way if you are on approach to an airport in a well lit city it can help to split the airport out from all the roads and highways when looking for it in the terminal area.

Large commercial aircraft have lots of instruments and navigation equipment but small private planes aren't so different from your car. If a plane has an emergency, you want it to be able to be able to tell where the airport is without the airport having to figure out that something is going on. Obviously, a pilot will tell the airport about an emergency if they can, but what if you have a small plane whose radio has died? They're flying around at a few thousand feet looking just like any other small plane, and there's no real way for the airport to figure out that somebody wants to land, if that somebody can't see the airport.


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How can you explicate the efficiency of a turbofan engine with Carnot's theorem?
From Carnot's theorem, we know that efficiency is
$$ \eta = 1-\dfrac{T_c}{T_h}$$
where $T_c$ is the temperature of the cold reservoir and $T_h$ the temperature of the hot reservoir. But what do we have to consider for both?
Is $T_c$ the temperature of the air coming into the compressor from the fan or the temperature of air coming into the combustion chamber after being compressed and heated by the compressor?
Is $T_h$ the temperature of the gases in the combustion chamber (1500C°) or at the exhaust nozzle?

The Carnot efficiency is the maximum attainable efficiency in terms of extracting work from a temperature differential, not the actual efficiency in a turbine engine. For the actual efficiency, you should look into the Brayton cycle, which approximates a turbine engine much better.
With that out of the way, let's look at the Carnot cycle. This cycle assumes a heat source that can provide arbitrary amounts of energy at a certain temperature $T_h$, and a heat sink that can sink arbitrary amounts of heat at a temperature $T_c$. These are the isothermal parts of the cycle. To get the working fluid to either temperature, adiabatic compression is used.
These heat sources and sinks do not exist in reality, but by approximation, the atmosphere is an excellent heat sink, and burning fuel is an excellent heat source. The low temperature is thus taken as the atmospheric temperature which can be as low as -50°C (~220 K, remember that Carnot's efficiency is defined in terms of absolute temperature) and the high temperature is the temperature in the combustion chamber.
Sometimes you hear that $T_h$ is the hottest temperature in the cycle, and $T_c$ the coldest temperature. While this is often true in practice, this does not need to be the case for theoretical considerations. The only thing that matters for the Carnot efficiency is the temperature of the heat sinks/sources. You can't get more efficient than the Carnot cycle by hooking your engine up to a fridge. Indeed, the Carnot cycle is just a way to express the Second Law of Thermodynamics, which states that entropy in a closed system can never increase.

I see. But if we, for example, consider the outside air temperature to be -50C° and the temperature in the combustion chamber to be 1500C° for the Carnot theorem we would obtain an efficiency over the value of 1, which is clearly not correct.
Looking at the Brayton Cicle, I see effciency depends on pressure ratios. But what pressures does it consider? Outside air pressure? Pressure in the compressor and combustion chamber, or at the exhaust nozzle?

@franycio Carnot efficiency is expressed in kelvin (or really, any absolute temperature scale), not in degrees Celsius. Try again with 223K and 1773K.

right, forgot that very detail. Thank you.

@franycio Regarding the pressure ratios, this is also based in theoretical considerations, namely, a heat sink at a low pressure and temperature, and a heat source at high pressure and temperature. So here too take the combustion chamber and the atmosphere pressures. Take a look at thermodynamic p-V diagrams and really try to understand what's happening and how that applies to reality (for example, the combustion chamber in a turbine does not tend to have a large pressure differential - it's just a chamber - so whatever process happens there must be approximated as isobaric).

@Sanchises For ICE engine what is  Tc, surouding atmosphere temperture? why this formula dont use  temperature of exhaust in consideration? lower exhaust the better

@22flower Remember that we're talking about the idealized Carnot cycle, not a real ICE engine. In the Carnot cycle, we don't discard heat at a higher temperature than the coldest temperature in the cycle. It's an idealized world where we actually manage to fully expand the exhaust to match the cold ambient air.

@Sanchises Is formula for effficiency for real IC engine OK?  e=1- (T4-T1)/(T3-T2)


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Why mount multiple autocannons on a fighter or CAS aircraft as opposed to a revolving barrel auto-cannon with a higher rate of fire?
Why would one design an aircraft with multiple chain guns instead of a single large rotary cannon, like the GAU-8 Avenger, and similar weapons?

ehum, what's "mini" about the GAU-8?

Image source
Same difference as between a shotgun and a rifle: if you need to hit a moving target, like a clay trap target or pigeon or aircraft, it's best to spread the projectiles out so that there is more of a chance that the target meets them. If however the target is (almost) stationary, nothing beats lining up carefully and inflicting the maximum damage with the projectile.
Heavy machine guns are mounted on swivels to create an effective area instead of a line, and only one bullet would be required to stop a soldier. A trap shooter swivels while overtaking the spinning disk, thereby spreading out the spray of shot. But an A-10 lining up for a tank kill wants to maximise impact upon this one tank only - just the one bullet landing anywhere on an armoured tank may not do the job.
When multiple cannons/machine guns are mounted across the wings, the aircraft is set up for dogfighting. Which they are not designed for anymore.

A gatling type gun takes a certain period of time to reach it's designed rate of fire. The M61 20mm gun takes about 0.5 second to reach 6000 rounds per minute, which means that during the first half second only about 25 rounds are fired, whereas a battery of four revolver cannon each firing at 1200 RPM would fire 40 rounds in the first 0.5 second.

The only advantage would be redundancy.  If one gun jams, you still have others which can fire.  The primariy disadvantage to a multi gun scheme is weight.   A single gun has one action, receiver, magazine,  and ammunition feed system, where is multiple guns will each require those components for each of them.   Multiple barrels on a single gun solves the problem of a rapid fire action while providing cooling of the barrels during operation.  Multiple guns also consume more space inside of the aircraft, which limits additional real estate for things like fuel, mission systems etc.
Additionally,  you’re not really using the right terms there. Miniguns were a trademark of General Electric, which developed a scaled down version of their rotary cannons chambered for the 7.62 x 51mm rifle cartridge for use on gunships.  GE also manufactured a variety of rotary cannons of varying calibers for use on aircraft, including the M-61A1, GAU-8/A, and others.

On a fighter, you'd mount multiple guns if the high speed rotary gun hadn't been invented yet, which was the case in WW2.
The rotary gun is lighter that multiple guns, takes up less space, has a higher rate of fire, and is more accurate than multiple guns firing at a slower rate.
WW2 guns were typically wing mounted, and all guns set to converge at a fixed point, typically around 200 yards. If the fighter pilot shot at a plane not at that point, a good deal of the bullets would miss.
Even with the few planes that had centrally mounted guns like the P38 and ME110, the recoil of the individual guns would cause their aim to diverge, though it wasn't as pronounced as wing mounted guns. Some F86 pilots in Korea added reinforcements to the mounts for their six Browning M2's in the nose, to keep them from moving so much while firing and improving their accuracy.
The single high speed gun puts all the bullets in the same place, regardless of distance. Given the difficulty in getting another aircraft in one's sights, the ability to inflict a much higher level of damage in a short period of time is a real advantage.
For CAS, especially helicopters, slower rates of fire stretch out the ammunition supply, for softer targets that don't require a huge amount of damage in a short period of time. After all, a soldier is just as dead whether they get hit by two bullets, or two hundred. Note that this is for dedicated CAS helicopters that are expected to loiter near the combat area for long periods of time.
When the potential target becomes hardened, like a tank, then the GAU8 at it's full speed does what the CAS pilot wants: puts the maximum destruction in one spot, in the minimum time.
Some helicopters, such as the MH53, come with a single 7.62 rotary gun in the back, but it's job is not continued CAS, but suppression when inserting or removing troops from a hot LZ, which is done as quickly as possible, and then the helicopter departs the combat zone. A stream of bullets from a rotary cannon tends to make soldiers seek cover, and stop firing at you.
One other CAS aircraft that uses rotary guns is the AC130. It is expected to loiter for long periods of time, but the AC130 can carry a lot of ammunition, so running out isn't as much of a concern as it is with a load limited helicopter.
Gun choice for ground troop support depends upon the expected mission.

BF 109, FW 190,  P61 black widow, P39, Mosquito, Beaufighter. Nose mounted guns during WW2 are not nearly as rare as you made out. The spread caused by having wing mounted guns is often considered an advantage. Pilots more likely to hit with a shotgun than a sniper rifle. For an expert the sniper rifle is better but for average joe with <20hrs going into combat the shotgun approach is going to be better


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Can current fighter jets perform a deadstick landing?
If a current modern day fighter were to have a total engine failure over a suitable landing site or airport, can the pilot safely perform a dead stick landing? If so, are they routinely given training for the procedure?

Here's an F-16 doing it.

Apparently there is an emergency APU that runs on hydrazine to power the hydraulics instead of a RAT.  They mention it at the end because they have to call out fire trucks because of the hydrazine hazard.
I used to have an F-104 flight manual, which I must have loaned out to someone and never saw again.  It could actually glide fairly well, with an L/D of about 4 or 5 to one at about 190 kt if I recall.  It had a section on gliding engine out.

Yes, in fact it has happened successfully at least once I can think of. In 1997 an air national guard pilot landed an engine out F-16 on the numbers. It's not the most advanced fighter out there but it shares almost all the characteristics of the newest as it's reliant on electrical power and computers to fly.
As for training, fighter pilots are routinely taught engine out procedures at least to the point of restart, I do not know if they teach dead stick landings though, although I suspect not.
There's a youtube video of the approach and landing here.

What does dead stick mean? No power?

Yes, it means that there is no engine power at all @Cloud. It comes from the early days of propeller engines IIRC, if the engine quit the stick (propeller) was dead, i.e. not moving.


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How to identify the faulty GPS-INS in dual redundant installation?
If we have two GPS-INS sensors on an aircraft, how can we decide which one is giving the correct measurement? I know that for  sensors, we can vote, but what if we have two sensors? Please give me a reference if there is one.

Each INS tracks its own computed position, not updated by GPS or other inputs; if one INS position diverges significantly from the other, then the GPS position could "break the tie" to identify which INS is navigating poorly (and needs to be replaced by maintenance).

Two is for redundancy in case of a complete failure - not to vote in case of false information.
For that you need more to decide in case something goes wrong (normally you would use an uneven number for that). But luckily there is more - like traditional navaids (VOR, DME, NDB, ...)
You can see that on most shorter haul aircraft (where these navaids are likely to be available, they use two systems (failure redundancy), while on long haul you commonly find three systems. But with more and more robust technology this also gets reduced.

If you want a decent solution that may still have availability or accuracy issues, there are two traditional ways of doing fault detection and exclusion: comparison between sensors, and comparing each sensor to a model.  You could use both simultaneously to detect a split, then identify which sensor is more likely to be faulty.
Often each sensor is compared to a system model, using statistical hypothesis testing like a chi-squared test, cumulative sum algorithms, or checking Kalman filter innovation values. This is called "model-based fault diagnosis" or "analytic redundancy" in literature. Extended Kalman filters are the industry standard for position and velocity, but you could also look at sliding mode observers, proportional integral observers, or even something much simpler, like an assumption that changes in speed are always below certain thresholds. This lets you exclude a single sensor without performing any cross-checks, but your exclusion is only as good as your model. Covering every practical scenario can be difficult and can give false positives when an INS realigns, satellites drop in and out of the GNSS solutions, and tests points jump around.
Multiple redundancy is traditionally done by assembling several sub-groups or sub-filters, to identify a sub-filter that has good performance.  This may be done using sensor models and statistical tests like when we're looking at a single sensor, or it might use the distribution expected from the split between sensors based on their reported HFOM or ANP. The problem in your case is that with only two sensors, you can't get a working combination of multiple sensors in the face of a fault. The only alternative I've seen before is to use a synthetic sensor for comparison, like extrapolating from the previous state using control inputs, but if the noise in that estimate is larger than the sensor faults you're trying to exclude this doesn't add anything.
There seems to be an assumption here you're the blended GPS-INS solution is the only source of position,  and not individual INS and GPS? In reality this isn't usually quite the case.  Yes, a "tightly coupled" INS that uses individual satellite data is usually better engineered to detect GPS faults than what you could do. On the other hand, there are lots of faults, like on ARINC 429, that you can mostly eliminate by simple comparisons between INS data with GPS. So in reality you rarely have only two position sources in a modern cockpit.
The short answer is that generally you can't exclude faults with only two GNSS-INS with a high level of confidence. Even if you had a good way of resolving ambiguous faults, if one of the systems fails now you have no more fault detection and the pilot can't trust the avionics. As explained further in this answer, while you can do CAT III with two INS, the availability of such a system has to be scrutinized to match safety targets. Less stringent uses like CAT I or EGPWS can use just two INS (or sometimes even one) just fine.

Hmmm... the 737 has had a Cat III autoland capability for decades with only 2 INS's. Both have to be working, but only 2 are installed.

Thanks @RalphJ, I've updated my answer accordingly


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Will I fail a check-ride if I do a go-around for a short-field landing?
Let's assume that I'm on a check-ride and my next requirement is to perform a short-field landing.
If I'm coming in for landing and it's obvious I'm not going to hit my mark, can I perform a go-around and try again? Would that be considered a fail?

At the risk of stating the obvious, if you are not going to make it and you don't go around you are also going to fail, but in a much more spectacular way.

In general, you get one free go-around.

First of all, any checkride which involves short-field landings probably also involves demonstrating go-arounds.  If you don't do one on your own, your examiner will have to ask to see one later.

Going around when it's obvious you're not going to make your point demonstrates good judgment.  It shows that you're not going to try to force the airplane into an unsafe situation just to hit your mark.  If the runway you're landing on really is a short field, continuing the landing attempt would not only be a bad idea, it would be unsafe, and the examiner would be correct in taking the controls from you, as well as failing you.  If the runway is long and you're simulating a short field landing, the same applies.

If you go around on a short-field landing and then go around on the next one as well, the examiner is going to start to wonder whether you really know what you're doing.  Let's face it, it should not take you three tries to get properly set up for a short field landing, unless perhaps if the day is remarkably turbulent or there is a strong gusty crosswind.  If that's the case, though, why in the world did you decide to take your checkride in those conditions?

This is almost verbatim what I tell students. It's true of any phase of the checkride, in my opinion. If you're in a situation you don't like, do what you would do if the examiner wasn't sitting next to you.

Yes I believe good judgement, aeronautical decision making, weighs heavier than making the perfect approach. You will need to demonstrate that you know how make a good short-field approach but you also need to demonstrate a go-around and my experience reflects that of Steve's in that the examiner will let you choose to go around on your own and count it towards that. Some will even purposely try to put you into a situation where you need to go around to see if you make the correct decision. Go-around is ALWAYS the better decision than a bad landing (unless you waited too long to clear trees).

"why in the world did you decide to take your checkride in those conditions?"  Maybe you didn't, and the conditions blew up once the checkride was already in progress?

In short, No.
I messed up both my short field, and soft-field landings.
I properly called and executed a go-around, and hit the landings on the second tries.
The examiner passed me, and gave me credit for executing properly when the landing couldn't be saved.
I think the important thing that he called out after the flight is that a Private Pilot's License is a "License to Learn" in a very literal sense.   You are not required to execute every maneuver perfectly to pass.  You are required to execute to a tolerance such that the safe outcome of the flight is not in question.
All instructors and examiners understand that learning is a life-long process, and recent events have repeatedly shown us how even the most experienced commercial pilots still need improvement on some skills.  As long as you demonstrate that you can control the plane, exercise good judgement, and keep everything safe, you're likely to pass.
The corollary to this is that once you have your license, you cannot believe that you are done and qualified.   Having a PPL just means you have a ton more to learn, but you can practice and learn a lot of that on your own, without an instructor constantly next to you.

No, you shouldn't fail if you're going around for a valid reason and the go-around is well executed. If you're well prepared, the way you operate on the checkride should be how you would operate if the examiner was any other passenger.
If you take a look at the Practical Test Standards for Airplane (FAA-S-8081-14B), there are a couple of areas that help answer your question. The biggest one is that Normal & Crosswind Landing (Task B, p. 43) includes this as a subtask:

the applicant [...] 11. Executes a timely go around decision when the approach cannot be made within the tolerances specified above.

...while Go-Around/Rejected Landing (Task L, p. 50) includes:

the applicant [...] 2. Makes a timely decision to discontinue the approach to landing.

That said, it's interesting to note that none of the other landing tasks include the "timely go around decision" subtask from Task B, so this possibly gives examiners a bit of leeway. You should be executing the go around as early as possible, at the soonest point you think that your setup is not correct. If you're too high on base, or you're not descending enough, or you forget to get your flaps in at the right time - reset the maneuver! And when you execute your go-around, make sure it is well executed (and to the PTS standards).
Ultimately, a lot of your checkride comes down to the criteria under Examiner Responsibility (p. 10):

Satisfactory Performance
Satisfactory performance to meet the requirements for certification is based on the applicant’s ability to safely:

perform the Tasks specified in the Areas of Operation for the certificate or rating sought within the approved standards;
demonstrate mastery of the aircraft by performing each Task successfully;
demonstrate satisfactory proficiency and competency within the approved standards;
demonstrate sound judgment and exercises aeronautical decision-making/risk management; and
demonstrate single-pilot competence if the aircraft is type certificated for single-pilot operations.

Between "mastery of the aircraft..." and "demonstrate sound judgment", there is a wide range left up to the examiner. Most examiners, I feel, are pretty sympathetic to students' nerves and the day's conditions. If it takes you one (maybe two) go-arounds to nail your short-field landing, and the rest of the flight went well, you're set. If they have to take over because you're constantly going around... you're going to get to try again later after some more practice.

I did a go around on the short field for my PPL checkride. I was floating too much and even with a slip and s-turn I couldn't make it. So I went around and tried again. On my second try I turned base further downwind so I had a long final. Then I dragged it in for a spot on landing. The DE asked me if I knew why I was floating so much and I told him I had a tailwind but on the surface the windsock showed totally calm. He said I was right and that he calculated a 15 knot tailwind while I was flying. There was a lot of other traffic in the pattern so I didn't have the option of switching sides.
The DE told me that the go around was the right thing to do because he can't fail me for my landing if I don't land. He said if I was doing a lot of go-arounds for no reason that would be a problem, but otherwise a go around isn't a landing.


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What is the name for yaw, pitch and roll together?
What is the name for yaw pitch and roll all together ?
I've heard CMD used to refer to all of that information but don't know what it stands for.
Is CMD correct? if so what does it stand for? is there some other word?

Related: What is the exact meaning of "attitude", does it include translational movement?

The questions seem clear to me: Is there a word for referring to the data set (yaw, pitch, roll)? Could that name be "CMD"? It could have been closed as duplicate, not unclear. The second question is probably a mix with autopilot mode CMD.

....bad airmanship?

@CarloFelicione I was going to suggest "a spin"  ;)  but that most likely isn't what is being asked about.

@mins I believe so. I added an answer to explain CMD.

Do you mean these three instantaneous values, or do you mean a maneuver that varies all three simultaneously?

Roll, pitch and yaw are referred to in one word as the attitude.

Pitch, roll and yaw are rotation angles relative to the earth gravitational field. In order to fully describe the state of an unrestrained rigid body in space, there are six Degrees of Freedom required. This site states it as follows:

4.1.2 Degrees of Freedom of a Rigid Body in Space
An unrestrained rigid body in space has six degrees of freedom: three translating motions along the x, y and z axes and three rotary motions around the x, y and z axes respectively.

The three linear co-ordinates determine the position of a reference point, usually the Centre of Gravity. The three rotary co-ordinates determine the orientation of the body, relative to the chosen axes.
So pitch, roll and yaw are the three rotational degrees of freedom of a rigid unrestrained body in space, determining the attitude of the body.

Others have already answered your question (the answer is attitude), but I would just like to talk about "CMD" This is an autopilot mode, it has nothing to do with attitude (well I guess it kind of does, read on!). There are two modes: CMD and CWS. CMD stands for "Command" and CWS stands for "Control Wheel Steering". I will use the Boeing 737 in this example:

This is part of the MCP (Mode Control Panel), which is the panel that controls the autopilot. These are the buttons you use to engage the autopilot. Here is the full MCP:
As you can see there are 4 buttons, two showing CMD and two with CWS (there are two of each because there are two autopilots, in case one fails, each set controls a different one). CMD mode will follow whatever you tell to autopilot to do. For example, if you set the heading to 100° and 10,000 feet, it will follow those instructions. In CWS mode, it acts kind of like the FlyByWire system of an Airbus. You use the yoke to control the plane, and the autopilot will adjust to maintain that attitude after you let go of the yoke.
More about these two modes: http://www.flaps2approach.com/journal/2015/11/6/control-wheel-steering-cws-explained.html
I hope this helps

In aerobatics, it is called a Lomcovak

What is the name for yaw, pitch and roll together?

Depending upon which of the maneuvers of the Lomcovak family the pilot chooses to perform ...

The aircraft will rotate on all three axes and perform three end-over-end negative "g" tumbles, each tumble being at about 45° to the plane of the last. (Main Lomcovak).

This amounts to changing pitch, yaw, and roll simultaneously.  The illustration of the three planes offered by Koyovis should help visualize what is being done.
Warning: this isn't a trivial maneuver to attempt
This maneuver can be rough on various structures in the aircraft, and for a pilot not well practiced in aerobatics can be very disorienting.  Don't try this unless you know what you are doing, and are in a suitably built aircraft.


text:
What are the loads in a horizontal stabilizer?
I am doing a project and I can't really find any complete information on how to estimate and calculate de loads (lift, weight, drag...) in a horizontal stabilizer.
If someone could give me good papers or books references would be very helpful.
Thanks in advance.

Remember, in the air, any load will produce torque, if it is not counter-acted, the tail simply weathervanes.  Start with a diagram of forces around the aircraft CG.

Also there are some excellent "primers" that help visualize how the tail is most often producing a down-force that offsets the arm that exists between the CP (Center of Pressure) and CG (Center of Gravity.)  This video is a good starting point to begin your exploration:  https://youtu.be/FNC5NHRv5KE  I'm not saying a video is a suitable reference for your project, rather, it is a jump-start to your intuition about why the tail does what it does.

Maybe you didn't see this advice that I have already given you, that should clarify all of your doubts

Sorry, but resource location questions are specifically off-topic here.

If you are into commercial transport aircraft, you might start with:
"Structural Loads Analysis for Commercial Transport Aircraft"
by Ted L. Lomax,
ISBN (print): 978-1-56347-114-8,
Chapter 8: Horizontal Tail Loads

The way I'd approach this problem may vary depending on what information is available about the aircraft and flight condition you are analyzing, but here is a rough outline of how I'd suggest approaching this problem.

Determine aircraft CG location and wing center-of-pressure. Both of these things should be fairly straightforward to determine, or reasonable approximations can be made. With this information, you can determine the aerodynamic force required of the horizontal stabilizer to balance the pitching moment. You can start with some assumptions to make the calculation more straightforward (e.g. the horizontal stabilizer is the only element contributing to the pitching moment) and then relax them as needed to improve the result depending on which ones are valid for the flight condition you are considering (e.g. in a gliding flight condition there would be no thrust contribution to the pitching moment.)
By balancing the moments you'll get a value for the lift produced by the horizontal stabilizer. To get drag, you'll want to determine the local angle-of-attack, as well as the elevator deflection required, which depends on the flight condition. If you have information about the horizontal tail airfoil, planform, and other geometry, that will be helpful too. You'll want the angle-of-incidence of the horizontal stabilizer and wing to determine the angle-of-attack at the horizontal stabilizer (again, start with restrictive assumptions about no downwash from the wing, and relax as needed.) You can consider the effect of a trim tab (if used) as well. This should start coming together to give you a value for drag.

Depending on why you are calculating these loads and how you are bookkeeping your aerodynamic forces, you can also consider interference effects.
In terms of resources, I learned a lot from Airplane Aerodynamics & Performance by Jan Roskam. If you get the book and continue reading the section below, it should reference other sections in the book to help you piece together all of the aerodynamic contributions you are interested in. There may be other better resources than this one, but this is simply what I am familiar with.


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Center of Pressure(CP)
The center of pressure is:

A)The point at which the Total Aerodynamic Force acts
B)The point where the lift force acts
C)The point where the resistance force acts

Only one answer is correct,i think is B or A but the question is really strange.

Welcome to aviation.SE. We would like to help you, but we need to understand where you're having a problem.
Why you're not sure between A and B? What doubts do you have? Why you think the question is strange?

Closely related, perhaps a dupe?

The fact that Total Aerodynamic Force is capitalized indicates it is a term that can be researched:
http:// www.dynamicflight.com/aerodynamics/aero_force
Total Aerodynamic Force includes drag, therefor the answer is B)
the point where the lift force acts.

Drag will determine the vertical position of the CP while lift is responsible for its lengthwise position. Since we only look at the lengthwise position for further analysis, it is sufficient to only use lift to determine the CP. But strictly speaking you need to look at both forces in order to pin down the CP location. +1

or, in other words, if you use Total Aerodynamic force (TAF), you would perform a Vector Calculation, considering the entire (vector representation) of the TAF, which, since it is a vector, has two components, a piece normal (perpendicular) to the Flight Path, (called LIFT), and a piece parallel to the flight path (called DRAG). Then you would do the calculation for each of these and combine the answers to get the 2-D position of the CP.  So if all you are interested in is the longitudinal (Horizontal) position of the CP, you simply use Lift, which is the vertical (normal) component of TAF.